Universal motors have a wound stator and a wound rotor with the rotor windings being electrically connected to the stator windings via brushes and a commutator. The windings may be connected in series (series wound) or in parallel (shunt wound). A universal motor, in theory, may run on DC power or single phase AC power. In practice, the universal motor is more suited for use with a single phase AC supply. Due to this kind of motor having high rotation speed, small volume, light weight, big starting torque, convenient speed control, and series excitation characteristic, it is widely used in medical devices, power tools, and electrical appliances, etc., which require low output power, high speed, small volume and light weight, such as vacuum cleaners, domestic sewing machines, electric drills, and other portable electric tools.
Universal motors have stators with either salient poles or non-salient poles. In conventional salient pole type universal motors, stator windings are wound about neck portions of the salient poles of the stator core. However, due to the neck portions of the stator pole being usually comparatively wide, each turn of the stator winding is long, which results in waste of materials. In conventional non-salient pole type universal motors, stator windings are wound on the yoke of the stator core which is comparatively narrow. However, the reluctance of the stator core is such that the magnetic field is not uniform across the poles and is concentrated in the portion of the pole closest to the stator windings with the other side of the pole carrying little magnetic flux. This results in an unbalance in the magnetic field which adversely affects motor performance, such as poor motor commutation, excessive sparking and reduced motor life. A salient pole may be described as a pole which clearly stands out from the yoke of the stator core. This is emphasized by the neck portion about which the stator winding is often wound. A non-salient pole is a pole which merges with the yoke and is difficult to determine where the pole begins and the yoke ends. The non-salient pole does not have a neck portion. As such the non-salient pole allows the yoke to locate closer to the rotor resulting in a smaller overall dimension of the motor, at least in the direction across the stator poles.